貓尾巴--音樂、音響

由 **drunkenlife** » 2013-02-08, 12:10

這題掛著很久了...
拖拖拉拉不如請大家提出各自的看法吧～

由 **drunkenlife** » 2013-02-12, 13:00

理想上，線，該等於無物，僅傳遞訊號，不加也不減，
這點該沒錯吧？

由 **drunkenlife** » 2013-02-22, 11:42

實務上，線必然不可能理想，多少有些偏頗，有些限制，甚至有些癖性...

由 **River** » 2013-03-05, 15:29

要談線恐怕先要談端子和界面 :lol:

燒友常不知覺執著於線的粗細材質結構以及各種神妙的線材理論
卻忽略了不可避免的接點與端子
以最常用到的RCA端子來說各位有興趣可以查一下巿面上結構合理真是用cupper來製作的公母頭有多少?
其實是屈指可數大部分的發燒線或是機器上的端子都是所謂的brass銅合金
而Brass的導電性只有銅的不到30%!! :shock:

先不談公母頭的接觸結構你用再好的XN純銅或銀線再加上奇妙的傳導理論都先在公頭的brass打個大折扣
然後還有母頭...再想想一條線兩邊都有頭哩!
以目前隨便上枱面的發燒線的價格我實在建議可以好好review一下自己的系統接點
那些錢恐怕可以把機器上的brass RCA母頭換好些遍了
你確定聽到了這些超級線材的實力? 哈! 這好比載著層層的耳塞來比較不同小提琴的聲音
慎之丫

由 **drunkenlife** » 2013-03-07, 11:29

說到接頭，說法也是一大堆啊...
鍍金？鍍銠？

無論是線本身或是接頭，該如何判定優劣？
最終當然是耳聽為憑，花錢之前，有啥方式能至少抓個大概？
看評論？這可能最難，
音響評論寫的都像武俠小說，裡面一堆無形劍氣高來高去，完全抓不出個頭緒..

由 **River** » 2013-03-09, 23:56

最近拿75 ohm同軸線當訊號線來用, 沒想到還非常好.....至少把我手上在用的老Siltech給全部換下來了
不同系統可能會有不同的結論.....給大家參考, 如果手上有75 ohm coaxial的不妨接起來試試看

由 **drunkenlife** » 2013-03-10, 21:38

River 寫:最近拿75 ohm同軸線當訊號線來用, 沒想到還非常好.....至少把我手上在用的老Siltech給全部換下來了
不同系統可能會有不同的結論.....給大家參考, 如果手上有75 ohm coaxial的不妨接起來試試看

嗯...這挺有趣的...
先來談談75 ohm同軸線是怎麼回事吧，

這75 ohm，是指這線的特性阻抗，
也就是若線末的負載是75ohm，那麼傳過去的訊號完全不會反射回來（阻抗匹配），（ viewtopic.php?f=4&t=452&p=2089&hilit=impedance#p2089 ）
例如電視的composite訊號是末端加個75 ohm的電阻落地。

也就是說，要真的利用這種線的優點，負載端的輸入阻抗必須是75 ohm，
否則送過去的訊號還是會反射回來，疊加在後續送達的訊號上。

由 **River** » 2013-03-11, 01:55

的確用75ohm同軸線在訊號傳輸上, 一開始我心裡也怪怪的 :roll:

但接上去, 用上合適的端子, 似乎又不得不承認在我的系統裡有效
也因此回頭來看BJC為何以75ohm coaxial cable使用在unbalance audio訊號傳輸上的技術觀點....
這裡節錄他的論點:
The funny thing about unbalanced audio, as we discovered when we entered the cable business, is that there's very little cable on the market which is expressly designed for use as unbalanced audio interconnect. Unbalanced audio doesn't see a lot of professional use, so many broadcast professionals often simply use whatever balanced audio cable they have on hand, and leave one of the conductors unconnected; meanwhile, though some manufacturers do make cable specific to unbalanced audio, it's often not really well-built for the application and is intended more for building "throw-in" style economy cables--poorly shielded, and high in capacitance.

These two attributes, shielding and capacitance, are the most important factors in unbalanced audio cable quality. Shielding is important, of course, because it keeps out externally-induced noise, and because unbalanced audio, unlike balanced audio, can't take advantage of common-mode noise rejection. Capacitance contributes to high-frequency rolloff, so the lower the capacitance of the cable, the flatter the frequency response in any given application (how flat will depend on the device impedances as well as the capacitance, so it's not possible to generate a one-size-fits-all frequency response chart; but in every case, the lower the capacitance, the flatter that curve will be). Capacitance, like many cable attributes, is a per-foot characteristic; while high capacitance won't ordinarily make a significant difference in short runs, it becomes an increasing problem with longer runs.

When we entered the cable business, the best cables we could find for unbalanced audio use were cables which had actually been designed and built for video. Why? Well, that's because video cables are coaxial, which is the right geometry for an unbalanced audio cable; because video cables are typically well shielded; and because video cables, being designed to a 75 ohm characteristic impedance, are relatively low in capacitance, ranging from about 16 pF/ft for an HDPE-foamed dielectric precision video cable (e.g. Belden 1694A) up to about 21 pF/ft for a solid PE dielectric cable (e.g. Belden 8281 or Canare LV-77S).

In testing various available cables against one another, we found that double-braid shield designs, where one braid shield is laid directly on top of another, provided the best low-frequency noise rejection. As it happens, conveniently, double-braid shielding is usually found on high-flexibility video cables, so the best noise rejection and the best flexibility for cable size were typically combined in the same products. As a result, we recommended Belden 1505F for general analog audio use, and Canare LV-77S (higher capacitance, but slightly better low-frequency noise rejection) for subwoofer use. These were, and remain, excellent products for this application; but we felt we could build something a bit better.

Video cables are constrained in their design by the need to maintain a characteristic impedance of 75 ohms. As we've mentioned, that generally locks capacitance in at 16 to 21 pF/ft. But analog audio cables don't need to maintain any particular characteristic impedance. Unbalanced audio cable is usually run from a low-impedance output into a high-impedance input; it's not an impedance-matched system like video, and it doesn't need to be, because the wavelengths of analog audio signals are so long that--barring audio cables that are miles long--impedance just doesn't matter. Because analog audio isn't an impedance-matched system, we can make the characteristic impedance of an analog audio cable come out to any value at all, without any adverse consequence.

By the way: we often hear from people on internet discussion boards, or in e-mails inquiring about our products, that "audio cable is supposed to be 50 ohms." There is indeed a lot of 50 ohm coaxial cable in the world, and no doubt some of it has been used for analog audio; but there is not now, and has never been, any standard impedance spec for unbalanced analog audio cable. 50 ohm cable isn't a good choice for analog audio. That's not because of the impedance, which doesn't matter at all, but because of the capacitance, which is quite high in 50 ohm cables (typically 25 to 31 pF/ft). We're not sure what the origin of the "50 ohm audio cable" myth is, but it doesn't seem to want to die.

Since impedance isn't a factor, and capacitance is, we aren't bound by the practical limits of capacitance for a 75 ohm cable. We are, however, bound by various other practical limits. If a cable gets too thick, it becomes inflexible, and incompatible with reasonably-sized connectors. If the center conductor is too thin, it becomes fragile, and becomes difficult to reliably attach to connectors. Capacitance, in a video cable, is a function of three things: outer diameter of the center conductor, inner diameter of the shield, and the dielectric constant of the material between them. Within reasonable limits, we wanted to optimize all of these factors to reduce capacitance: small center conductor, large shield diameter, and low-density foamed dielectric.

Meanwhile, we wanted to maintain a very high shield effectiveness, particularly at low frequencies. To do that, we chose the shield design used on two large high-flex cables: Belden 8281F and Canare LV-77S. Both of these cables use a heavy double-braid shield, and the Canare LV-77S tested best for hum rejection in our comparison of a number of well-shielded cables. The 8281F has a tinned shield, while the LV-77S has a bare copper shield. There's no significant difference in performance between the two; however, we decided to go with the bare copper braid because it's marginally more conductive than tinned copper.

The dimensions of the cable were, ultimately, dictated by practical connectorization; we went with a 25 AWG solid copper center conductor because that was as small as we felt we could make it while maintaining (1) the ability to firmly crimp a center pin to it and (2) adequate strength for durability under pulling force. The shield dimensions of the LV-77S/8281F configuration, meanwhile, not only were as large as we wanted the cable to get from a flexibility standpoint, but also were compatible with crimping to the largest Canare RCA connector bodies, which are made for LV-77S.

For the dielectric, we wanted a material as foamy and soft as possible; solid polyethylene, like that in LV-77S and 8281F, was not good because its dielectric constant is relatively high and would keep capacitance up. Air, of course, is ideal as a dielectric, but one can't build a coax using only air; foamed polyethylene is the answer, because it combines the mechanical stability of PE with the best dielectric: air. We went with Belden's low-density PE foam.

The result is a cable which combines, to the extent practical, the best possible attributes of an analog audio line-level cable. Its capacitance is extremely low, at 12.2 pF/ft, while its shielding is extremely effective at rejecting audio-frequency interference. Meanwhile, though it has the outer dimensions and appearance of 8281F, it is more flexible due to the extremely soft dielectric and smaller center conductor. Theoretically, one could improve further on both shielding effectiveness (by inserting more layers of shielding) and capacitance (by increasing the size of the dielectric, shields and jacket)--but either would require making the cable impractically large. We feel that LC-1 represents the best combination of electrical characteristics and usability of any analog audio cable on the market today.

他使用75ohm coaxial的理由, 其實並非是impedance matching, 而是75 ohm coaxial有夠好的shielding以及夠低的capacitance. 這才是他要的.
其實LC-1這條cable用上原配的端子, 只能說是一般, 但也的確誤有所值. 這位老兄應該也並非發燒一族..... :lol: